This invention relates to processes for reducing the oxygen content of metal oxide materials and has one application in producing nuclear fuels.
The use of uranium and plutonium oxide fuels in nuclear reactors is well known. Such oxide fuels may be in pellet or granular form. It is also known to use fuel which is an intimate mixture of these oxides, or a mixed oxide, produced, for example, by processes which involve co-precipitating uranium and plutonium compounds from solution, or by blending powders of the two oxides. For some purposes it is desirable for such oxide fuels to contain less than the stoichiometric amount of oxygen, eg in the oxide fuel represented by (PuU)O.sub.x, that x should be less than 2.
Such a fuel having x less than 2 can be produced by forming the mixed oxide or mixture of oxides with x approximately 2 or greater than 2, and reducing this material in a hydrogen-containing gas stream. For example, a material represented by (PuU)O.sub.x, where x is greater than 2, can be considered for present purposes as a mixture of PuO.sub.2, UO.sub.2 and U.sub.3 O.sub.8. A fuel desired to have x=2 can be produced by reducing this material in such a gas stream, when the U.sub.3 O.sub.8 component tends to be reduced to UO.sub.2. Similarly a fuel having x less than 2 can be reduced by further such reduction, the PuO.sub.2 component being reduced to Pu.sub.2 O.sub.3 (the UO.sub.2 component is not reducible to U.sub.2 O.sub.3 in this way).
In the course of such reductions, water is formed by reaction between the hydrogen and the oxide or oxides, and the presence of this water tends to inhibit the reaction. It has now been discovered that, using a particular type of carbon-crucible furnace in which there is contact between the gas stream and the carbon of the crucibles, the water is apparently removed by reaction with the latter carbon (to form carbon monoxide and hydrogen), thereby allowing the required reduction to proceed.